AutoDiag

P0400

Exhaust Gas Recirculation "A" Flow

Moderate· Fix soonIntermediatePowertrainGeneric, all makes

What it means

The ECU has detected a problem with exhaust gas recirculation (EGR) flow. The EGR system feeds a small amount of exhaust back into the intake to lower combustion temperature and NOx emissions.

Is it safe to drive?

Usually safe to drive. Fix to avoid engine pinging and failing emissions.

Symptoms

  • Check engine light on
  • Pinging or knocking under load
  • Rough idle if stuck open

Probable causes

Most likely first

  1. 1Carbon-clogged EGR valve or passages
  2. 2Faulty EGR valve
  3. 3Vacuum or wiring problem to the EGR valve
  4. 4Faulty EGR position or pressure sensor

How to diagnose it

Work through these in order before buying parts

  1. 1Record the code and its freeze frame data (speed, load, temperature when it set), then clear it and see if it comes back.
  2. 2Command the EGR valve with a scan tool and watch engine RPM and MAP: idle should stumble when it opens.
  3. 3Remove the valve and inspect for carbon.
  4. 4Check for technical service bulletins (TSBs) or software updates for your make and model; some codes have known factory fixes.

Common fixes

  • Clean the EGR valve and passages
  • Replace the EGR valve

This information is general guidance for all makes. Exact causes, specifications and procedures vary by vehicle, so confirm with your service manual or a qualified mechanic before replacing parts.